In a DRAM principally used heretofore, a memory cell includes an MOS transistor and a capacitive element (capacitor) and write or read of information in or from the memory cell is effected by storing or detecting electric charge in the capacitor by switching operation of the MOS transistor.
A stacked capacitor, in which a second electrode (upper electrode) is formed so as to be superposed on a first electrode (lower electrode) with a dielectric film interposed between them, is usually used for this capacitor.
However, in DRAM memory cells having stacked capacitors thus constructed, it has become difficult to obtain a desired storage capacitance of the capacitor because of a decreased space applicable to the stacked capacitor in each cell with increasing integration density and fineness of the semiconductor memory device.
Therefore it is important to increase the storage capacitance of the capacitor formed in a restricted space. In order to solve this problem, a method has been proposed to increase the storage capacitance of the capacitor, in which an effective capacitor area is increased by forming fine projections on the upper surface of the lower electrode acting as a storage node, as disclosed in International Electron Devices Meeting IEDM (1990) pp 659-662.
By this method, an amorphous silicon thin film is deposited while controlling temperature and pressure for forming the thin film at depositing the silicon thin film serving as the lower electrode on a substrate by the CVD method to form the fine projections on the upper surface of this thin film.
However, in the case where the lower electrode of the stacked capacitor is formed by using the method described above, since fine projections formed on the upper surface of the thin film are reduced in size when the thickness of the thin film is increased to about 0.2 .mu.m, which is a practical thickness, it is necessary to form the thin film of a desired thickness by repeating the process for forming a film of a smaller thickness several times. Further, since the magnitude of the fine projections depends sometimes on the temperature at the formation of the thin film, precise temperature control within .+-.5.degree. C. is required. For this reason and others, in the method described above control of the thin film forming process is complicated and difficult.